Drop on demand inkjet printing systems eject ink drops from printhead nozzles in response to pressure pulses generated within the printhead by either piezoelectric devices or thermal transducers, such as resistors. The ink drops are ejected toward a media sheet where each ink drop forms a spot on the media sheet. The printheads have a plurality of inkjet ejectors that are fluidly connected at one end to an ink supplying manifold through an ink channel and at another end to a nozzle, sometimes called an aperture, in a nozzle or aperture plate.
In some phase change or solid ink printers, known as direct printers, the printer ejects ink drops directly onto a print medium such as a paper sheet. After ink drops are printed on the print medium, the printer moves the print medium through a nip formed between two rollers that apply pressure and optionally heat to the ink drops and print medium. One roller, referred to as the “spreader roller” contacts the printed side of the print medium. The spreader roller is heated and coated with a release agent that prevents ink drops on the print medium from transferring onto the spreader roller. The second roller is referred to as a “pressure roller.” This roller presses the media against the spreader roller. The pressure roller may be optionally heated to facilitate the fixing of the ink to the sheet of print medium. The heat and pressure applied through the nip flattens the ink drops and secures the printed ink image to the print medium in a process known as “fixing.”
In an indirect printing embodiment, the printer ejects ink drops onto an intermediate member such as a rotating drum or endless belt. A pressure roller referred to as a “transfix” roller is positioned against the intermediate member to form a transfix nip. A media sheet passes through the transfix nip, and the ink image on the intermediate member transfers and fixes to the media sheet under pressure and heat in the transfix nip. The transfer and fixation of the ink image is referred to as a transfix process that is well known to the art.
Both direct and indirect inkjet printers are capable of producing either simplex or duplex prints. Simplex printing refers to production of an image on only one side of a print medium. Duplex printing produces an image on each side of a media sheet. In duplex direct printing, an ink image is formed on a first side of the media sheet and then the sheet passes through the spreader nip to fix the ink image onto the first side of the media sheet. The medium is then inverted and sent along a path that passes the second side of the media sheet by the printheads for the formation of a second ink image on the second side. The sheet then returns to the spreader nip where the second ink image is fixed to the second side of the media sheet. A similar process is used with indirect printing, except the image is initially formed on an intermediate drum and then transferred to the media and in an indirect system the image is fixed at the same time as the ink is transferred.
In both direct and indirect printing systems having significant levels of oil on the media before imaging is undesirable since the release agent can prevent ink from properly adhering or transferring to the media. Therefore, preventing the release agent from transferring to the back side of a sheet during printing of the first side image is desirable. Current printing systems must slow down and use special sequencing in duplex mode to prevent release from being transferred to the back of a sheet during front side printing. If the spreader roller and the pressure roller in the direct printer contact one another before a media sheet reaches the nip or between sheets as they pass through the nip, then release agent transfers from the spreader roller to the pressure roller. In an indirect printer, the transfix roller and intermediate member also contact one another before a media sheet reaches the transfix nip or between sheets as they pass through the nip and release agent transfers to the transfix roller from the intermediate member.
The amount of release agent on the pressure roller may reach a level that causes release agent to be transferred above the allowable limit from the pressure roller to the second side of a media sheet while an image is being fixed to the first side of the media sheet. For duplex prints, the excessive release agent transferred to the second side of the media sheet may interfere with the printing of ink images on the second side of the media sheet. In a direct printing system the presence of release agent between the ejected ink and the sheet may result in the transfer of some ink from the media sheet to the spreader roller during the fixing of the second image on the media sheet. In an indirect printing system, the presence of release agent on the media may result in some ink remaining on the intermediate member instead of transferring to the media. In both cases, the loss of ink produces an image having partial or missing pixels. When the partial or missing pixels are detectable by the human eye the phenomenon is known as image dropout.
One way of addressing the buildup of release agent on the pressure roller is to keep the two rollers separate from each other until a media sheet is presented for the fixing operation. That is, the one roller can be selectively positioned with respect to the other roller. Thus, if the movable roller comes into contact with the other roller as the media sheet approaches the rollers, then little or no release agent is transferred from the spreader roller to the pressure roller. This synchronization, however, requires sophisticated control techniques and, during high speed duplex printing, the movement of the one roller can become unwieldy or a limit to the throughput of the printer. Consequently, improved operation of direct and indirect printers that addresses this limitation would be beneficial to higher throughput.